Whisk egg yolks and sugar together until pale yellow and creamy
Dissolve coffee & cocoa powder in a tablespoon of hot water, stirring. Pour into egg mixture with melted butter, and beat until well combined.
Sift flour and baking powder into a bowl and fold in with wooden spoon or with the stand mixer's paddle attachment.
In a separate bowl, whisk the 6 egg whites & cream of tartar on high until they form stiff peaks. Fold into mocha batter, until just incorporated.
Divide into 2 prepared 8-inch cake pans. Place each pan onto a baking sheet.
Bake at 180 degrees centigrade, for 20-25 minutes or until a bamboo skewer inserted into the middle comes out with just a few crumbs attached.
Cool on a wire rack, for 10 minutes, before removing completely from the baking pans to cool on wire rack.
Cool completely (even overnight) before frosting.
Make the frosting:
Set your mixing bowl in a pan of simmering water (for ease of use, use your stand mixer's bowl) and whisk egg whites and sugar for 3 to 5 minutes, until fluffy, marshmallow-like and hot. You may need a separate hand-held electric hand mixer for this.
Remove bowl from heat, scrape the handheld's whisks free of marshmallow meringue, and then using your stand mixer, beat the meringue for another 5 minutes with the whisk attachment.
Add half the butter and beat until smooth. (You may opt to use the paddle attachment for this, or stay with the whisk attachment) Add the rest of the butter, beat until smooth again. Scrape sides, stir buttercream mixture quickly, then go back to beating for 6 to 10 minutes.
Dissolve cocoa, coffee and vanilla extract in a very small amount of hot water, stirring vigorously.
Add to buttercream and beat on high for a further 2-3 minutes, scraping as needed.
Use to put a layer of buttercream between the cakes, and to frost and decorate the cake.